¹⁸F-FDG PET/CT imaging in tendon xanthomatosis

Excerpt

A 27-year-old woman presented with multiple non-tender nodules on both elbows and knuckles. The patient has a history of familial hypercholesterolaemia, and a pattern of cholesterol levels was compatible with hyperliopproteinaemia type IIa: raised total cholesterol (425 mg/dL), markedly raised low-density lipoprotein (LDL, 278 mg/dL), nearly normal high-density lipoprotein (33 mg/dL) and normal triglycerides (50.9 mg/dL). The patient underwent ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography (PET)/CT, which demonstrated multiple increased FDG activities in the tendons of both elbows, hands, knees and feet (Figure) with the maximum standardized uptake value of 5.0. Excision biopsy of the mass in the left elbow was performed for diagnosis. Histopathologic examination showed aggregates of foamy macrophages, confirming the diagnosis of xanthoma. Tendon xanthomas are pathognomonic deposits of lipid and connective tissue commonly found in patients with severe hyperlipidaemia, as occurs in familial hypercholesterolaemia [1]. The elevation in serum LDL cholesterol levels secondary to the LDL receptor defect produces oxidized or otherwise modified LDL, and the uptake of oxidized LDL by the macrophage scavenger receptors results in massive lipid accumulation and foam cell formation [2]. Monocyte-derived foam cells due to intracellular accumulation of oxidized LDL, extracellular unesterified and esterified cholesterol and connective tissue are the main components of tendon xanthomas [3, 4]. It seems that increased ¹⁸F-FDG activity of xanthoma is due to dense macrophages. The high uptake of ¹⁸F-FDG by macrophages may be the result of two important factors: (1) Macrophages have relatively high metabolic rates, and (2) macrophages do not store glycogen and thereby rely on external glucose as a source of fuel for the hexose monophosphate shunt pathway [5, 6]. When activated, macrophage glucose consumption increases further still. In this case, the ¹⁸F-FDG PET/CT could clearly identify multiple tendon xanthoma lesions during a single whole-body examination, and as far as we know, this is the only case demonstrated metabolic activities of tendon xanthomas by ¹⁸F-FDG PET/CT imaging. The role of ¹⁸F-FDG PET/CT imaging in evaluation of tendon xanthomas needs further investigation.

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